As the power density of the power supply is increased, the demand on the operating efficiency is gradually increased. That is, the operating frequency of the power conversion circuit of the power supply needs to be enhanced. Take a power conversion circuit operated in a pulse width modulation (PWM) mode for example. Generally, as the switching frequency is increased, the size of the power supply is reduced but the switching loss is increased.
On the other hand, the resonant power conversion circuit may have higher conduction losses of switch when compared with the power conversion circuit in the PWM mode. With increasing development of the switch elements, the conduction resistances of the switch elements are decreased and thus the conduction loss is limited. Furthermore, since the power conversion circuit in the resonant mode could provide zero voltage switching or zero current switching, the switching loss is reduced and the operating efficiency at the high frequency is enhanced. As the switching frequency is increased, the size of the power conversion circuit is reduced. As a consequence, the applications of the resonant power conversion circuit are expanded. The resonant mode power conversion circuit is an important solution to achieve high frequency, high power density and high efficiency.
FIG. 1 is a schematic circuit diagram illustrating a conventional resonant DC-to-DC power conversion circuit. As shown in FIG. 1, the DC-to-DC power conversion circuit comprises a switching circuit A1, a resonant network A2, a transformer Tr and a filtering and rectifying output circuit A3. The switching circuit A1 comprises multiple switch elements for converting an input voltage source Vin into a high-frequency pulse voltage. The high-frequency pulse voltage is applied on the resonant network A2 which comprises a resonant inductor Lr and a resonant capacitor Cr. As such, the AC voltage across the primary winding of the transformer Tr transfer the electrical energy to the filtering and rectifying output circuit A3 through the secondary winding of the transformer Tr, thereby generating an output voltage Vo. The filtering and rectifying output circuit A3 comprises an output capacitor Co, a rectifying switch element Sw (e.g. a MOSFET or diode) and optionally an output filtering inductor Lo. In addition, the magnetizing inductance and the leakage inductance of transformer could be considered as portions of the resonant network A2. Take a LLC circuit for example. If the switching frequency is lower than the resonant frequency of the LLC resonant network, the magnetizing inductor of the transformer operates in resonant mode. In other words, the resonant network includes the magnetizing inductance of the transformer.
FIG. 2 is a schematic circuit diagram illustrating a conventional single-phase half-bridge LLC resonant power conversion circuit. In the power conversion circuit of FIG. 2, the switch elements at the primary side (e.g. S1 and S2) are turned on in a zero voltage switching (ZVS) manner and turned off in a resonant manner. In a case that the switching frequency is lower than the resonant frequency fr of the LLC resonant network and higher than fm, the diodes D1 and D2 at the secondary side will be turned off in a zero current switching manner, wherein
            f      r        =          1              2        ·        π        ·                                            L              r                        ·                          C              r                                            ,          ⁢            f      m        =          1              2        ·        π        ·                                            (                                                L                  r                                +                                  L                  m                                            )                        ·                          C              r                                            ,and Lm is the magnetizing inductance of the transformer Tr. The magnetizing inductance could also be obtained by connecting an external inductor with the primary winding of the transformer Tr in parallel. Since the power conversion circuit of FIG. 2 could be soft switched, the switching loss is very low. This circuit has been widely used in LCD-TV, notebook computer, telecom device or server because of its simple configuration.
Although the single-phase half-bridge LLC resonant power conversion circuit has some benefits, there are still some drawbacks. For example, according to the increasing of the power level, especially the increasing of output current, the ripple current of output filter increases a lot and thus the ripple of the output voltage Vo is increased. For reducing the output voltage ripple, the capacitance of the output capacitor Co needs to be increased. Alternatively, a complicated two-stage filter circuit at the output side is another solution for the same purpose. The means of reducing the ripple according to the prior art, however, increases the number and volume of the components and increases the overall cost of the power conversion circuit.
As the output current Io is increased, the ripple contained in the input current Iin is increased. For reducing the ripple contained in the output current and the ripple contained in the input current, a two-phase half-bridge resonant DC-to-DC power conversion circuit has been disclosed in for example EP1331723A2. In the DC-to-DC power conversion circuit of the European patent EP1331723A2, the control signals for controlling the switch elements at the primary winding side have 90-degree phase shift. In addition, the switch elements have the same switching frequency.
As the demand on the power is increased, the two-phase resonant DC-to-DC power conversion circuit is insufficient to reduce the ripple contained in the output current and the ripple contained in the output voltage. Recently, a three-phase resonant DC-to-DC power conversion circuit was disclosed for increasing the efficacy of reducing the ripples contained in the input and output currents.
FIG. 3 is a schematic circuit diagram illustrating a conventional three-phase half-bridge LLC resonant power conversion circuit. The input sides of the first phase circuit P1, the second phase circuit P2 and the third phase circuit P3 are connected in parallel. The output sides of the first phase circuit P1, the second phase circuit P2 and the third phase circuit P3 are connected in parallel. Except the connection between the input sides and the connection between the output sides, there's no additional connection between the first phase circuit P1, the second phase circuit P2 and the third phase circuit P3. In addition, the first control signals S1a, S2a, S3a and the second control signal S1b, S2b, S3b of the first phase circuit P1, the second phase circuit P2 and the third phase circuit P3 are respectively complementary to each other. The phase shift between any two adjacent first control signal S1a, S2a and S3a is 120 degrees. The phase shift between any two adjacent second control signal S1b, S2b and S3b is 120 degrees.
In a case that the power conversion circuit is applied to a high-power electronic device, the parameters of corresponding components need to be same in order to obtain the same magnitude of output currents in all of the three phase circuits. That is, the capacitance values of the resonant capacitors Cr1, Cr2 and Cr3 are same, the inductance values of the resonant inductors Lr1, Lr2 and Lr3 are same, and the magnetizing inductance values of the Lm1, Lm2 and Lm3 are same.
For mass-producing the components of the power conversion circuit, the components have respective tolerances. For example, the tolerance between the nominal inductance value and the real inductance value of an inductor is usually ranged from −15% to +15%. In addition, the tolerance between the nominal capacitance value and the real capacitance value of a capacitor is ranged from −20% to +20%. The increase of the precise will increase the fabricating cost. Due to the tolerances of the component parameters (e.g. inductance values, capacitance values or the like), the resonant frequencies of 3 phase circuits are different. If the tolerances of the component parameters are too large, the variations of the resonant properties of the phase circuits are increased.
FIG. 4 is a schematic timing waveform diagram illustrating related current signals processed in the three-phase power conversion circuit as shown in FIG. 3. Due to the tolerances of the component parameters (e.g. inductance values, capacitance values or the like), the peak values of the first phase current i1, the second phase current i2 and the third phase current i3 are distinctness. As such, the different phase current brings different current at the primary side of the transformer and different current at the secondary side of the transformer. Under this circumstance, the power loss of the power conversion circuit is increased and the operating efficiency thereof is reduced, even a component fail of the circuit happens.
For solving the above drawbacks, a power conversion circuit was disclosed is for example Japanese patent No. JP200178449, which was filed by Sanken on Mar. 23, 2001. FIG. 5 is a schematic circuit diagram illustrating a power conversion circuit disclosed in Japanese patent No. JP200178449. As shown in FIG. 5, a first coupling inductor L12, a second coupling inductor L22 and a third coupling inductor L32 are respectively connected to the resonant networks of the first phase circuit, the second phase circuit and the third phase circuit in series. The first coupling inductor L12, the second coupling inductor L22 and the third coupling inductor L32 are coupled with each other. By means of the first coupling inductor L12, the second coupling inductor L22 and the third coupling inductor L32, the current-sharing efficacy of the power conversion circuit is enhanced. Since each phase circuit needs an addition component, the operating efficiency of the power conversion circuit is reduced, and the overall volume of the electronic device having the power conversion circuit is increased.
Therefore, there is a need of providing an improved multi-phase switching power conversion circuit so as to obviate the drawbacks encountered from the prior art.